Organosiloxane compositions, which cure to elastomeric solids, are well known. Typically, such compositions are obtained by mixing a polydiorganosiloxane having reactive terminal groups, generally silanol groups, with a silane cross-linking agent which is reactive with the polydiorganosiloxane, for example an acetoxy silane, an oximosilane, an aminosilane or an alkoxysilane. These materials are frequently curable upon exposure to atmospheric moisture at room temperature.
One important application of the above-described curable compositions is their use as sealants. In use as sealants, it is important that a composition is capable of curing in comparatively thick layers to provide an elastomeric body having a thickness greater than about 2 mm. It is frequently desirable that the composition cures quickly enough to provide a sound seal within several hours but not so quickly that the surface cannot be tooled to desired configuration shortly after application. Among specifically desirable attributes for such compositions are fast surface cure rate, good elasticity of the skin formed and lack of surface tackiness after curing for about 24 hours. End users are known to consider that a sealant with a residual surface tackiness has several drawbacks, for example, it may be thought that the product will not cure completely and thereby, may lead to the, typically incorrect, belief that such a problem also causes poor bulk properties. However, tacky surfaces are more prone to dirt pick up in a dusty environment such as in a factory. In addition, it has long been desired to have a sealant, which retains its color or translucency, and lack of color after curing and having a sealant surface at the sealant air interface which does not pick up dirt over time.
In order to achieve the desired speed of cure of hydroxy or hydrolysable polymers using alkoxysilane cross-linkers, it has become general practice to employ tri or tetra alkoxy silanes as cross-linkers in combination with organic tin or titanium compounds as condensation reaction catalysts. It is generally understood within the industry that a cross-linking agent must contain at least three reactable or cross-linkable groups so that the agent has the ability to link multiple chains together. Titanium compounds most generally preferred for this purpose are those derived from primary, secondary or tertiary alcohols, for example, isopropyl alcohol and n-butyl alcohol. The titanium compounds used are often employed in combination with a chelating agent such as an acetyl acetonate as an accelerator and stabilizer for the titanium compound.
The goal of providing a user with a cured end-product having minimal or no after cure dirt pick-up and/or staining has been the subject of a wide variety of patent publications. Hence, it will be appreciated that this is a long known problem recognized within the silicone sealant industry. As will be seen below a wide number of expensive and complex additives and compositions have been proposed to provide a “clean” sealant with minimal or no dirt pick up. In many of these cases the introduction of such additives, whilst providing a reduction in dirt pick-up, have other negative effects on the cured end-product such as staining and discoloration and as such a simple proven means of providing a “clean” sealant remains a goal of the sealant industry. The following prior art discusses previous attempts to solve this difficult and well known problem.
WO 01/42365 describes a silicone sealant composition having an organopolysiloxane comprising on average from greater than 1 to less than 2 reactive sites per molecule; a polyfunctional organosilicon comprising one or more hydrolysable groups per molecule and at least one functional group capable of reacting with the reactive sites of the polymer, a condensation cure catalyst and a filler. Typically the polymer reactive groups are silicon bonded hydrogen, hydroxyl or alkenyl groups and the organopolyfunctional organosilicon compound has the structure R4Si where at least one R group is hydrogen, alkoxy or alkenyl and at least one other R group is alkoxy, oximo, amino, aminoxy or acyloxy. However, whilst an enormous range of products fall within the scope of this definition, the standard compounds are specifically listed, i.e. preferred examples of the organopolyfunctional organosilicon compound include vinyltrimethoxysilane, tetramethoxysilane, methyltriethoxysilane, tetraethoxysilane, methyltrimethoxysilane or a mixture thereof This selection of silane based cross-linkers teaches the man skilled in the art that either 3 or 4 alkoxy groups are required for the invention. Furthermore, the catalyst can be any suitable condensation catalyst including tin, lead, antimony, iron, cadmium, barium, manganese, zinc, chromium, cobalt, nickel, aluminum, gallium or germanium as well as titanium and zirconium. The resulting sealant product is said to reduce staining.
U.S. Pat. No. 5,432,218 describes an organopolysiloxane having terminal silicon bonded hydroxyl or hydrolysable groups, a silicon containing cross-linker having at least two silicon bonded hydrolysable groups and the reaction product of an unsaturated fatty acid, such as tung oil, with a diorganopolysiloxane containing at least one silicon bonded amino, epoxy or hydroxyl group. These products give low dirt pick-up however products of these compositions tend to discolor, substantially due to the presence of the additives in the formulation.
U.S. Pat. No. 5,733,960 describes a means of increasing the longevity of low dirt pick-up type silicone sealant compositions by introducing zinc oxide into a composition containing an organopolysiloxane having silicon bonded hydroxyl or hydrolysable groups, a hydrolysable cross-lining agent, typically a ketoximosilane having at least 3 ketoximo groups per molecule, a tin or titanium based catalyst and tung oil.
U.S. Pat. No. 5,326,816 describes an organopolysilethylenesiloxane, organosilane having more than two hydrolysable groups, exemplified by a methyltributanoximo silane in combination with a UV absorber and/or an antioxidant. This composition is said to have good low dirt pick-up properties although the reason for the latter properties was unknown to the inventors.
EP 541074 A (Sunrise Meisei Corp) claims a 1-component sealant composition which uses a silicone-modified polyether together with a copolymer of at least two methacrylic esters in a solvent and a solid long chain primary or secondary saturated mono or di amine having a melting point of 40 to 100° C. in the same solvent. The latter two components are said to form a barrier layer on the sealant to reduce dirt pick up. The composition also contains a filler, an olefin, epoxy, methacryl, amino or mercapto functional silane cross-linker and a tin based catalyst.
U.S. Pat. No. 4,618,646 describes a sealant composition comprising a silanol terminated diorganopolysiloxane, a silane cross-linker having, on average, 2.01 hydrolysable groups per molecule, examples include methyltrimethoxysilane and vinyltris(N,N-diethylaminoxy)silane, silica fillers and silicone modified polyoxyalkylene compounds. The latter are provided to prevent sag.
U.S. Pat. No. 5,357,025 provides an elastomeric sealant composition, which is provided with a siloxaphobic surface layer (comprising a fluorocarbon compound) at the air sealant interface. The siloxaphobic surface layer comprises a fluorocarbon compound which contains per molecule at least one fluorocarbon group of the formula CxF(2x+1) where x has an average value of at least 6; and (ii) a drying oil oxidation product. The siloxaphobic surface layer is said not to become dirty and prevents staining.
U.S. Pat. No. 5,264,603 describes a sealant composition, the preparation of which involves the mixing of a silanol terminated polydimethylsiloxane with a mixture of hydrolysable silanes. The resulting sealant is designed to produce a thin protective layer at the air sealant interface by incorporating a mixture of an at least 6 carbon fluorocarbon compound and a drying oil such as Tung oil.
It can be seen from the proposals above that previously there has been an understanding in the art that complex compositions involving a wide variety of additives are required to solve this problem. However, in some instances the additives used can have negative side effects on the resulting sealants, such as enhanced discoloration of the cured product. Hence, an inexpensive and simple means of obtaining a cured silicone sealant which does not pick up dirt is still being sought.
The present invention seeks to provide an improved “clean” sealant which provides the end-user with a modified surface avoiding dirt—pick up and which after curing is a translucent, “water white” or white cured product which avoids the aesthetically unpleasant yellowing discoloration issue.